[Molecular genetics and diagnosis of retinoblastoma. Significance for ophthalmologic practice]

Retinoblastoma (RB) is initiated by loss of function of both copies of the retinoblastoma susceptibility gene (RB 1). Hereditary predisposition to RB is caused by germline mutations in the RB 1 gene. Tumor formation is initiated by the somatic loss of the second allele. Most patients with hereditary RB develop multiple tumors that usually affect both eyes. In nonhereditary disease, however, both RB 1 mutations are somatic events that cause the formation of a single tumor focus. Knowledge of the germline mutation is often essential for accurate risk prediction. Applying strategies for efficient mutation detection, germline mutations can be identified in most individuals with hereditary RB. The vast majority of mutant alleles cause premature termination of translation owing to frameshift or nonsense mutations. In patients carrying these mutant alleles, penetrance is almost complete (> 95%) and numerous tumor foci are observed. However, some 5% of the mutations result in comparatively mild alterations at the protein level. Patients with mutations of this kind often show a lower mean number of tumor foci (reduced expressivity) or no tumor at all (incomplete penetrance). Reduced expressivity and incomplete penetrance are also observed in patients with large cytogenetic deletions. By mutation analysis in DNA from fresh frozen tumor samples and peripheral blood, we have detected RB 1 germline mutations in some 20% of patients with unilateral RB. These results emphasize the importance of molecular analysis in patients with isolated unilateral RB.